Tiltable Stool

ABSTRACT

An improved tiltable stool comprises a seat, a body structure, and a base. The base comprises an annular elastic base member having a downwardly convex outer surface and a downwardly concave inner surface. Deformation of the annular elastic base causes a stabilizing force which pushes the tiltable stool towards a normal position when the tiltable stool or chair is tilted out of the normal position. The stabilizing force increases with the weight of a user and provides a consistent dynamic seating experience for users of different weights. An upper section of the annular elastic base is substantially cylindrical and firmly connected to the base structure. The downwardly convex outer surface of a lower section of the annular elastic base member rests on the floor. A contact area between the annular elastic base member and the floor is substantially ring-shaped, and grows outwardly with an increasing weight placed on the seat.

TECHNICAL FIELD

The present invention generally relates to an article of furniture, andmore particularly, to a tiltable stool or chair which self-adjusts tothe weight of a user.

BACKGROUND

Articles of furniture such as stools or chairs which allow a user torock forward, backward and sideways are generally known. A tiltablestool is typically configured to be used on a generally horizontalsurface such as a floor. The stool comprises a top section providingseat and a base section comprising a rounded bottom surface configuredto support the stool on the floor.

The base section may be a weighted base which has a downwardly convexlateral surface area to support the stool upon contacting the floor whenthe stool is tilted out of its normal upright position. In that normalposition the stool rests on the floor with a flat or concave area of itsbase. The weighting of the base is so chosen that the center of gravityof the stool comes to lie inwardly of the perimeter of the centralcontact area in its tilted state. The support areas may be contiguous,thus forming part of a continuous annular surface, or may beperipherally spaced apart, as by being individually disposed on three ormore legs projecting generally radially from the base. Such stools aregenerally described in U.S. Pat. No. 3,312,437 and in US patentapplication publication US 2013/0320727.

The conventional stools typically assume an upright normal position whenunoccupied. The upright position is obtained by a resetting force whichacts on the stool when tilted out of its normal upright position. Theresetting force is caused by coordinating the center of gravity of thestool with the fixed shape of its base such that the center of gravityassumes its lowest position when the stool is upright. Typically, theresetting force is selected based on a desired characteristic of anunoccupied stool.

The rounded base of conventional stools have several disadvantages: Theymay cause noise when the stool is tilted, they require a relativelylarge and heavy base. The base may slide or roll away due to a smallcontact surface with the floor, and the stool generally providesinsufficient support for a user when tilting out of the uprightposition, making it undesirable or even dangerous in particular forelderly users.

Attempts have been made to address the inherent disadvantages of afixedly formed rounded base by using an inflatable base. An exemplaryseating arrangement having an inflatable rubber ring is disclosed inU.S. Pat. No. 6,644,742. The inflatable base requires occasionalreinflation, which is not practical. It may also be prone to outgassingand cause an undesirable odor.

Also, bases made of foam have been proposed, but those do not addressthe lack of support for a user to maintain a generally upright seatingposition and do not adjust to a user's weight.

SUMMARY

An improved tiltable stool provides soft and comfortable dynamic seatingwithout jeopardizing safety and stability. The stool is intended to beused while keeping both of a user's feet on the ground. When tilted outof a normal position the stool provides a stabilizing force which aidesin maintaining a stable seating position. The stabilizing force of thestool increases approximately exponentially with the tilt angle of thestool out of the normal, typically upright, position. The stabilizingforce also increases with the weight of a user. At a given tilt anglethe stabilizing force increases approximately linearly with the weightof the user, thus making the stool self-adjust to the user's weight. Theimproved stool provides a similar seating experience for both light andheavy users: All users can easily tilt out of the normal position, whileexperiencing a stabilizing force with increasing tilt angle thatcorresponds to the user's weight. The improved tiltable stool providesdynamic seating flexibility similar to that of an exercise ball, buteliminates the inherent instability of sitting on a ball. Exercise ballshave been associated with severe injuries when users have lost theirbalanced and fallen over backward. When the stool is unoccupied thestabilizing force is small and affected only by the weight of the stool,but sufficient to return a tilted stool into a normal position.

The improved stool comprises a seat, a body structure, and a base. Thebase comprises an annular elastic base member having a downwardly convexouter surface and a downwardly concave inner surface. The annularelastic base member is held in a base structure. The body structureextends between the seat and the base. Deformation of the annularelastic base causes a stabilizing force which pushes the tiltable stooltowards a normal position when the tiltable stool or chair is tilted outof the normal position. An upper section of the annular elastic base issubstantially cylindrical and firmly connected to the base structure.The downwardly convex outer surface of a lower section of the annularelastic base member rests on the floor. When the stool is upright acontact area between the annular elastic base member and the floor issubstantially ring-shaped, and grows outwardly with an increasing weightplaced on the seat.

Preferably, the annular elastic base member has a taperedcross-sectional shape with downwardly decreasing thickness. Typically,the normal position of the stool is upright. When the stool is uprightthe outer surface of an upper section of the annular elastic base memberis substantially vertical. However, the normal position may also beselected such that the stool is biased out of the upright position.

A tongue-and-groove connection may be used to connect the annularelastic base member to the base structure. A circular tongue at theupper end of the annular elastic base member engages a correspondingcircular groove of the base structure. The annular elastic base may bepress-fitted, glued, welded, or mechanically fixed to the basestructure. In particular, an electrically conductive disc may bedisposed within the circular groove of the base structure. The annularelastic base member may be welded to the base structure by applying anelectric current through the electrically conductive disc.Alternatively, the tongue of the annular elastic base may be insertedinto the circular groove of the base structure by cooling the annularelastic base to reduce the width of the circular tongue.

The annular elastic base may be made of various elastic materials, andis preferably made of plastic, which may be reinforced by glass fibers.In particular, the annular elastic base may be made of thermoplasticpolyurethane (TPU).

In alternative embodiments the stool may comprise an annular elasticbase having an outwardly convex, substantially “j”-shaped cross section.Deformation of the annular elastic base affects a stabilizing forcewhich increases with a tilt angle between a tilted seating position andthe upright position. The stabilizing force may increase approximatelyexponentially with the tilt angle. The stabilizing force increases,preferably linearly, with a weight that is applied to the seat.

In yet another alternative embodiment the annular elastic base has asubstantially “o”-shaped cross section. The annular elastic base maythen comprise a pressurized cavity between an outer wall and an innerwall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective top view of an exemplary tiltable stool.

FIG. 2 is a perspective bottom view of the stool as in FIG. 1.

FIG. 3 is a cross sectional view of an exemplary tiltable stool, showingthe stool in a tilted position.

FIG. 4 is a detailed cross sectional view of an exemplary annularelastic base under various loads.

FIG. 5 shows diagrams illustrating the relationship between load anddeformation of the annular elastic base and between tilt angle andstabilizing force.

FIG. 6 is a cross sectional view of an alternative profile of an annularelastic base under two selected loads.

FIG. 7 shows a “j” profile of an annular elastic base under variousloads.

FIG. 8 shows an “o” profile of an annular elastic base under variousloads.

FIG. 9 shows exemplary contact areas of the annular elastic base withthe floor under various loads.

FIG. 10 is a cross section view through an exemplary annular elasticbase in a normal (upright) position and in a tilted position.

FIG. 11 is a perspective bottom view of an exemplary annular elasticbase with additional ridges.

FIG. 12 is a perspective bottom view of an exemplary annular elasticbase in a tilted position.

DETAILED DESCRIPTION

Referring to FIGS. 1, 2 and 3, a tiltable stool 1 comprises a seat 2, abase 3, and an elongated body structure 4 between the base 3 and theseat 2. The stool 1 may comprise a height adjustment mechanism includingan adjustment lever 7 to adjust the length of the body structure 4. Thebody structure 4 may comprise a pillar assembly and defines a verticalaxis 13 of the stool 1.

The base 3 may comprising a base structure 6 connected to an annularelastic base member 5. The annular elastic base member 5 is configuredto rest on the floor 14. The stool 1 is tiltable in any direction bydeforming the annular elastic base member 5. When a tilting force isapplied to the seat, the seat is moved from a normal position into adynamic seating position. Typically, the normal position is upright. Inthe upright position the vertical axis 13 of the stool 1 isperpendicular to the floor 14. In response to a tilting force theannular elastic base member 5 is deformed, and the vertical axis 13 ofthe stool 1 is tilted by a tilt angle α out of the normal position. Theannular elastic base member 5 may be rotationally symmetrical and extendaround a central opening.

FIG. 3 shows a cross section of the stool 1 in a tilted seatingposition. Here, the stool 1 is tilted to the right by a tilt angle α outof the upright position. The annular elastic base member 5 is deformedand the height of the annular elastic base varies. As shown, the annularelastic base member 5 is compressed in the direction of the tilt (rightside of FIG. 3) and expanded opposite the direction of the tilt (leftside of FIG. 3). Compression of the annular elastic base member 5 causesa stabilizing force Fs at the seat 2 which counteracts the tiltingforce. In a stable tilted position the weight of the user Fw iscountered by an axial force Fa in the direction of the vertical axis 13and the stabilizing force Fs which is perpendicular to the axial forceFa.

The annular elastic base member 5 extends from a substantially circularupper end 38 to a lower end 39 around an opening. The diameter of theannular elastic base member 5 at the upper end 38 is larger than thediameter of the annular elastic base member 5 at the lower end 39.Beneficially, the diameter at the upper end 38 is between approximately1.2 and 1.6 times that of the diameter at the lower end 39. The annularelastic base member 5 has a downwardly convex outer surface 10 and adownwardly concave inner surface 11. The outer surface 10 of the annularelastic base contacts the floor 14 at a contact area 15. The contactarea 15 extends at a distance around the vertical axis 13. A tongue 36may be formed at the upper end 38 of the annular elastic base member 5which engages a corresponding groove 37 in the base structure 6 tofirmly connect the annular elastic base member 5 to the base structure6.

FIG. 4 shows the deformation of an exemplary annular elastic base member5 under various loads in more detail. When a first force F1 ofapproximately 400 N/m is applied, the annular elastic base member 5 hasa first height h1 and a first cross sectional shape 311. The first forceF1 correlates to a person weighing about 40 kg sitting on a stool towhich an annular elastic base member with a diameter of 320 mm isattached. As the stool is more heavily loaded, the annular elastic basemember 5 is deformed. When loaded with a second force F2 ofapproximately 800 N/m, corresponding to a person with a weight of 80 kgsitting on the stool, the annular elastic base member assumes a secondcross sectional shape 312. The height of the annular elastic base isreduced by a first deformation d2 to a second height h2. When furtherloaded with a third force F3 of approximately 1200 N/m, corresponding toa person with a weight of about 120 kg sitting on the stool, the annularelastic base member 5 assumes a third cross sectional shape 313. Underthe third load F3 the height of the annular elastic base is reduced by asecond deformation d3 to a third deformed height h3.

The relationship between deformation d and load F of an annular elasticbase is generally shown in FIG. 5. The relationship 500 betweendeformation d and load F is non-linear. More specifically, withincreasing deformation d the load F grows approximately exponentially.The non-linear relationship between load F and deformation d allowsusers of vastly different weight to use a stool equipped with theannular elastic base, without significantly affecting the overall heightof the stool.

Tilting a stool by an angle α out of the normal position causes astabilizing force opposite the direction of tilt. The relationshipbetween tilt angle α and stabilizing force Fr is generally shown incurves 501, 502 and 503 shown in FIG. 5. As illustrated, the stabilizingforce is zero when the stool is in an upright position, allowing a userto easily tilt. The stabilizing force Fr increases with increasing tiltangle α as shown in curves 501, 502 and 503. The different curves showthe stabilizing force for users of different weight. As shown, the firstcurve 501 illustrates the stabilizing force for a heavy user weighing120 kg. The second curve 502 illustrates the stabilizing force for amedium user weighing 80 kg. The third curve 503 illustrates thestabilizing force for a light user weighing 40 kg. At a given tilt angleα′ the stabilizing force F_(r1) experience by the heavy user is largerthan the stabilizing force F_(r2) experienced by the medium user, whichin turn is larger than the stabilizing force F_(r3) experienced by thelight user. This desired effect provides a similar seating experiencefor users of various weights. It provides additional stabilizing supportfor heavier users without impeding the ability of lighter users to reachthe same tilt angle.

FIG. 6 shows an alternative cross sectional shape 601, 602 of theannular elastic base member 5 in an unloaded state and in a loadedstate. The annular elastic base member 5 preferably has an inwardlycurved cross sectional shape 601, 602. The annular elastic base member 5may be tapered, having a downwardly decreasing thickness. The thicknessof the annular elastic base member 5 may decrease from a first thicknesst1 at an upper, substantially cylindrical section 325 of the annularelastic base member 5 to a lower thickness t2 at a lower end section 326of the annular elastic base member 5. The tapered profile of the annularelastic base member 5 supports the desired load/deformationcharacteristic as shown in FIG. 5. Thinner portions of the annularelastic base member 5 close to its lower end 39 bend more easily thanupper portions close to its upper end 38.

The uneven thickness of the annular elastic base member 5 allows a stoolequipped therewith to be used equally by a very light user, such as achild, and a heavy adult. In case of a child, only the lower, thinner,portions of the annular elastic base member 5 will bend. When used by anadult, the thicker, upper, portions of the annular elastic base member 5will also bend. In both cases the user will experience a similar “feel”of the stool's stabilizing force.

Under extreme load the lower end 39 of the annular elastic base member 5may bend through completely and come to rest against the base structure6. This limits the maximum deformation of and prevents damage to theannular elastic base member 5 when exposed to extreme loads.

The annular elastic base member 5 may be shaped outwardly convex andinwardly concave. The downwardly convex outer surface 10 and adownwardly concave inner surface 11 meet at the lower end 39 of theannular elastic base member. The lower end 39 of the annular elasticbase member may extend into a stiffening ring 40 as shown in FIG. 4. Thestiffening ring 40 at the lower end 39 helps stabilizes the annularelastic base member 5 at the contact area 15. The stiffening ring 40protects the lower end 39 from kinking when the stool 1 is loaded and/ortilted out of its normal position. The stiffening ring 40 is preferablyan integral component of the annular elastic base member 5.

To prevent kinking, the lower end 39 of the annular elastic base member5 may also be bent upwardly, such that the contact area 15 between theannular elastic base member 5 and the floor is below and radiallyoutward of the lower end 39 of the annular elastic base member 5.

When a stiffening ring 40 is used to strengthen the lower end 39 of theannular elastic base member 5, the unloaded annular elastic base 5 maycontact the floor at the stiffening ring 40 as is shown in FIG. 4. Thisarrangement has been found beneficial when working with relatively thickmaterial. When using relatively thinner cross sectional shapes it hasbeen found beneficial to provide the unloaded contact area 15 of theannular elastic base 5 radially outwardly and below the stiffening ring40 as is shown in FIG. 10.

Opposite the lower end 39 an upper end 38 of the annular elastic basemember 5 may be formed as a tongue 36 which engages a correspondinggroove 37 in the base structure 6. The tongue 36 may be slightly widerthan the groove 37 and during assembly the tongue 36 may be press-fittedinto the groove 37 with the help of a tool that secures the annularelastic base member 5 during insertion into the base structure 6. Thetongue may be further secured with self-tapping screws. As shown in FIG.11, mounting holes 12 may be provided and are preferablycircumferentially spaced at the base structure 6 to secure radiallyoutwardly directed screws into an upper portion of the inner surface 11of the annular elastic base member 5.

Alternatively, the tongue 36 of the annular elastic base member 5 may beassembled to the groove 37 of the base structure 6 by gluing or welding.In particular, the annular elastic base member 5 may be ultrasonicallywelded to the base structure 6. Welding may also be achieved byinserting an electrically conductive disc into the groove 37 or bymolding an electrically conductive element into the tongue 36. Anelectric current may then be applied to the electrically conductive discin order to cause resistive heating. The electric heating causes thesurface of the tongue 36 to meld and weld to groove 37.

Yet another assembly option is to cool the annular elastic base member 5to a temperature significantly below room temperature, causing thetongue 36 to shrink. Cooling may for example be affected by directing acold gas onto the tongue 36 just before insertion into the groove 37.The groove 37 may be dimensioned such that the cooled tongue 36 can beeasily inserted thereto, but is firmly held within the groove 37 oncethe tongue 36 warms back up to room temperature, expanding within thegroove 37. Experiments have shown that load on the stool 1 reinforcesthe tongue-and-groove connection between the annular elastic base member5 and the base structure 6, so that reinforcement of the connection bywelding or gluing is not critical and may not be necessary at all.

The cross-sectional profile and the material of the annular elastic basemember 5 are coordinated to provide a desired seating experience. Theannular elastic base member 5 may be made of thermoplastic polyurethane(TPU), rubber, thermoplastic polyolefin (TPO), fiberglass enforcedpolyamide (PA) or fiberglass enforced polyurethane (PU). The selectionof material requires a trade-off decision between cost andfunctionality. Experiments including durability tests have shown, that athermoplastic polyurethane with 90 Shore hardness provides the requiredrobustness at an affordable price. An annular elastic base member 5 madeof softer TPU with 75 Shore hardness would require about twice theamount of material as one made of TPU with 90 Shore hardness.

The following configuration of the annular elastic base member 5 hasbeen found to be particularly beneficial for users having a weightbetween 40 kg and 150 kg, which is a typical market requirement:

-   -   Material: TPU    -   Mass: 600 grams    -   Hardness: 90 Shore    -   Diameter at the upper end 38: 333 mm    -   Diameter at the lower end 39: 245 mm    -   Height h1 without load: 56 mm (including the tongue 36)    -   Height without load: 48 mm (not including tongue 36)    -   Tongue 36 dimensions: 8×8 mm    -   Thickness t1 at an upper section 325: 13 mm    -   Thickness t2 at a lower section 326: 2 mm    -   Thickness (diameter) of the stiffening ring 40: 8 mm

An alternative substantially “j”-shaped cross-sectional profile 701,702, 703 of an annular elastic base member 5 under three different loadsis shown in FIG. 7. This profile may be used in combination with hardermaterials such as fiber-enforced polyamide or polypropylene. The shown“j”-shaped profile 701, 702, 703 has a more even thickness andconsequently bends more readily compared to the tapered profile 601shown in FIG. 6. While this is generally not preferred as it increasesthe risk of kinking, the “j”-shaped profile 701, 702, 703 may bemanufactured more cost effectively, and may hence be a viablealternative for cost sensitive products.

Yet another alternative cross-sectional profile 801, 802, 803 issubstantially “o”-shaped as shown in FIG. 8. An annular elastic basemember 5 having a substantially “o”-shaped cross-sectional profile 801may be manufactured by extrusion, and thus be an alternative to moldedprofiles. Alternatively, the annular elastic base member 5 having asubstantially “o”-shaped cross-sectional profile 801 may be formed byrotational molding or by blow molding. The substantially “o”-shapedprofile 801 may comprise a cavity 804 which may be filled with air oranother gas and may be pressurized.

FIG. 9 illustrates the shape of the contact area 15 of the annularelastic base 5 with the floor 14 under various loads. An unloaded stoolmay have a contact area 15 that assumes a first circular shape 331. Forreference, the position 335 of the upper end 38 of the annular elasticbase member 5 is shown in a dashed line in FIG. 9. When the stool issymmetrically loaded and upright the contact area grows outwardly butremains circular. Consequently, the contact area 15 may assume a secondcircular shape 332 that has larger diameter than the first circularshape 331 when additional weight is placed on the stool. When tilted,the contact area may assume a substantially oval shape 333. As shown,the substantially oval shaped contact area 333 has a longer inner leverl_(i) than outer level l_(o). The inner level l_(i) is measured from theintersection of the vertical axis 13 with the floor 14 to the contactarea in the direction of the tilt. The outer lever l_(o) is measuredfrom the intersection of the vertical axis 13 with the floor 14 to thecontact area in the direction opposite the tilt. The contact area 15remains within the confines of the position 335 of the upper end 38 ofthe annular elastic base member 5 under all loads, and whether loadedsymmetrically or asymmetrically.

When the an annular elastic base member 5 is used with additionalcircumferentially spaced radially extending ridges 42, the contact area15 consists of a plurality of circumferentially spaced radiallyextending surfaces 336 that are arranged in an approximately circularshape 331, 332 or approximately oval shape 333 as explained above.

The stool may be tilted beyond its dynamic seating envelope of about 10degrees. In that case the substantially oval shaped contact area 333opens up at the inner end and eventually assumes an approximatelycrescent-shape 334. When tilting the stool beyond its dynamic seatingenvelope a user will remain a stable position by applying force to hislegs. Advantageously, the so tilted stool does not have a tendency toroll sideways. Further, the elasticity of the annular elastic baseprovides good friction on the floor and thus prevents the stool fromsliding backward.

When in its upright position the annular elastic base member 5 has asubstantially ring-shaped, circular, contact area 15 with the floor.Increasing deformation of the annular elastic base member 5 causes thediameter of this ring-shaped contact area 15 to grow outwardly. Thisincreases the effective lever arm 1 of the annular elastic base member5. When the stool 1 is tilted out of its normal position the contactarea 15 changes from a circular shape toward an approximately ovalshape. Beneficially, the effective inner lever l_(i) in the direction ofthe tilt is growing larger, while the effective outer lever l_(o)opposite the direction of tilt is getting smaller. This effect amplifiesthe stabilizing force of the stool and contributes to its stability.

The shape of the contact area as shown in FIG. 9 is further illustratedin the cross sectional view of FIG. 10. Here, the annular elastic base 5is shown in a symmetrically loaded or upright position 8 and in a tiltedposition 9. In the tilted position 9 the annular elastic base member 5is tilted by an angle α out of the upright position 8. The comparison ofthe symmetrically loaded upright position 8 with the tilted position 9shows the relative change of the inner lever l_(i) and the outer levell_(o). As shown, the inner lever l_(i) grows in the direction of thetilt and the outer lever l_(o) becomes shorter in the direction oppositethe tilt.

The maximum elastic deformation of the annular elastic base member 5 mayallow spring-loaded tilt of the stool of up to about 10 degrees and beassociated with the equivalent of a symmetrical load of 200 kg. A usermay tilt the stool beyond its dynamic seating envelope up to about 45degree. This is achieved by lifting the backward portion of the annularelastic base member 5 into the air. Tilting the stool up to about 45degrees allows a user to conveniently pick up articles from the floor.The shape of the annular elastic base member as described beforeprovides a relatively smooth and seamless transition from dynamicdeformation (up to approximately 10 degrees) to lifting the backwardportion of the base into the air (between approximately 10 degrees and45 degrees).

FIG. 11 shows an annular elastic base member 5 with additionalcircumferentially spaced radially extending ridges 42 at the contactarea of the annular elastic base with the floor. The radially extendingridges 42 have been found to prevent noise that might otherwise becaused by deformation of the annular elastic base member 5. Noise maystem from vibrations that may be caused when the annular elastic basemember 5 deforms while a user tilts the associated stool. The radiallyextending ridges 42 may also prevent a vacuum from forming under a stoolequipped with the annular elastic base member 5. This is particularlyrelevant if the annular elastic base member 5 and the base structure 6form an upwardly sealed surface. In that case, the annular elastic basemember 5 and the base structure 6 could act a large suction cup, whichmust be prevented. Prevention is achieved by the ridges 42 which segmentthe contact area of the annular elastic base 5 with the floor 14 andallow air to pass through air channels between the ridges 42. Instead ofridges 42, grooves (not shown) may be applied to the annular elasticbase member 5 in an equivalent circumferentially spaced and radiallyextending arrangement to provide the desired air channels. FIG. 11 showsa configuration with 44 ridges. Alternative configurations may choose touse more or fewer ridges, for example between 20 and 80 ridges.

FIG. 12 shows a deformed annular elastic base member 5 under extremeload. The lower end 39 of the annular elastic base member 5 here bendsthrough completely and comes to rest against a stop surface 320 of thebase structure 6. This limits the maximum deformation and therebyprevents damage of the annular elastic base member 5 when exposed toextreme loads. The extremely loaded stool remains slightly elastic basedon compression of the material of the annular elastic base 5. Damage tothe floor is prevented as the base structure 6 does not contact thefloor even under extreme load.

While the present invention has been described with reference toexemplary embodiments, it will be readily apparent to those skilled inthe art that the invention is not limited to the disclosed orillustrated embodiments but, on the contrary, is intended to covernumerous other modifications, substitutions, variations and broadequivalent arrangements that are included within the spirit and scope ofthe following claims. For example, while this specification and theclaims refer to a stool, it should be understood that the invention canequally be applied to a chair or other tiltable article of furniture.

Alternative Embodiments

A tiltable stool, comprising:

-   -   a seat;    -   an annular elastic base member; and    -   an elongated body structure extending between the seat and the        base.

The tiltable stool as above, wherein the annular elastic base isrotationally symmetrical.

The tiltable stool as above, wherein the annular elastic base membercomprises a downwardly convex outer surface and a downwardly concaveinner surface extending around a central opening.

The tiltable stool as above, further comprising a height adjustmentmechanism.

The tiltable stool as above, wherein the elongated body structurecomprises a pillar assembly and defines a vertical axis of the stool.

The tiltable stool as above, wherein the annular elastic base member isan integral molded component.

The tiltable stool as above, wherein the annular elastic base memberrests on the floor.

The stool as above, wherein the stool can be tilted in any direction bydeforming the annular elastic base.

The stool as above, wherein the stool can be tilted in any direction bydeforming the annular elastic base within a dynamic seating enveloped ofapproximately 10 degree tilt.

The stool as above, wherein the stool can be tilted beyond the dynamicseating envelope by lifting a backward portion of the annular elasticbase into the air.

The stool as above, wherein the stool wherein the stool has an uprightnormal position.

The stool as above, wherein deformation and/or compression of theannular elastic base member causes a stabilizing force.

The stool as above, wherein the stabilizing force increases with theweight of a user.

The stool as above, wherein the stabilizing force at a given tilt angleincreases approximately linearly with the weight of a user.

The stool as above, wherein the stabilizing force increases with thetilt angle of the stool.

The stool as above, wherein the stabilizing force increasesapproximately exponentially with the tilt angle of the stool.

The stool as above, wherein the height of the annular elastic basemember decreases in the direction of tilt when the stool is tilted.

The stool as above, wherein the height of the annular elastic basemember increases in the direction opposite tilt when the stool istilted.

The stool as above, wherein a tongue is formed at an upper end of theannular elastic base member.

The stool as above, wherein the tongue engages a corresponding groove inthe base.

The stool as above, wherein the tongue has a height of approximately 8mm and a thickness of approximately 8 mm.

The stool as above, wherein the tongue is press-fitted into the groove.

The stool as above, wherein the stool is configured to support a userhaving a weight between 40 kg and 150 kg.

The stool as above, wherein the annular elastic base is configured tosupport a load between approximately 400 N/m and 1500 N/m.

The stool as above, wherein the annular elastic base member has aninwardly curved cross section.

The stool as above, wherein the annular elastic base member is taperedhaving a downwardly decreasing thickness.

The stool as above, wherein the annular elastic base member is taperedhaving a downwardly decreasing thickness.

The stool as above, wherein the annular elastic base member is made ofthermoplastic polyurethane.

The stool as above, wherein the annular elastic base member has a massof approximately 600 grams.

The stool as above, wherein the annular elastic base member has ahardness of approximately 90 Shore.

The stool as above, wherein the annular elastic base member has adiameter at the upper end of approximately 333 mm.

The stool as above, wherein the annular elastic base member has adiameter at the lower end of approximately 245 mm.

The stool as above, wherein the annular elastic base member has adiameter at the upper end that is approximately 1.4 times its diameterat the lower end.

The stool as above, wherein the annular elastic base member has adiameter at the upper end that is between 1.2 and 1.6 times its diameterat the lower end.

The stool as above, wherein the annular elastic base member has heighthi without load of approximately 56 mm including a tongue.

The stool as above, wherein the annular elastic base member has heightwithout load of approximately 48 mm, not including a tongue.

The stool as above, wherein the annular elastic base member has athickness t1 at its upper end of approximately 13 mm.

The stool as above, wherein the annular elastic base member has athickness t2 at its lower end of about 2 mm.

The stool as above, wherein a lower end of the annular elastic basemember touches a stop surface when fully deformed under heavy load.

The stool as above, wherein deformation of the annular elastic base islimited by a stop surface in the base which supports the lower end ofthe annular elastic base under heavy loads.

The stool as above, wherein the annular elastic base is secured to thebase by screws.

The stool as above, wherein the annular elastic base is secured to thebase by self-tapping screws.

The stools as above, wherein mounting holes are provided andcircumferentially spaced at the base structure to secure radiallyoutwardly directed screws into an upper portion of the inner surface ofthe annular elastic base member.

The stools as above, wherein the lower end of the annular elastic basemember is bent upwardly, such that the contact area between the annularelastic base member and the floor is below and radially outward of thelower end of the annular elastic base member.

The stool as above, wherein a stiffening ring is provided at the lowerend of the annular elastic base member.

The stool as above, wherein the stiffening ring has a diameter ofapproximately 8 mm.

The stool as above, wherein the annular elastic base member contacts thefloor at the stiffening ring when the stool is unoccupied.

The stool as above, wherein the annular elastic base member contacts thefloor radially outwardly and axially below the stiffening ring when thestool is unoccupied.

The stool as above, wherein the annular elastic base member has anapproximately circular contact area with the floor when the stool isupright.

The stool as above, wherein the radius of the approximately circularcontact area with the floor increases as weight is placed onto thestool.

The stool as above, wherein the contact area of the annular elastic basemember with the floor is approximately oval when the stool is tilted.

The stool as above in a tilted position, wherein an outer lever l_(o)measured from the intersection of the vertical axis with the floor tothe contact area in the direction opposite the tilt is shorter than aninner lever l_(i) measured from the intersection of the vertical axiswith the floor to the contact area in the direction of the tilt.

The stool as above, further comprising circumferentially spaced radiallyextending ridges at the contact area of the annular elastic base withthe floor.

The stool as above, comprising between 20 and 80 ridges.

The stool as above, further comprising circumferentially spaced radiallyextending grooves at the contact area of the annular elastic base withthe floor.

The stool as above, comprising between 20 and 80 grooves.

What is claimed is:
 1. A tiltable stool, comprising: a seat; a basecomprising an annular elastic base member having a downwardly convexouter surface and a downwardly concave inner surface; and a bodystructure extending between the seat and the base, wherein the annularelastic base member has a tapered cross-sectional shape with downwardlydecreasing thickness and wherein deformation of the annular elastic basecauses a stabilizing force which pushes the tiltable stool towards anormal position when the tiltable stool or chair is tilted out of thenormal position.
 2. The tiltable stool as in claim 1, wherein the basefurther comprises a base structure, and wherein an upper section of theannular elastic base member is substantially cylindrical and firmlyconnected to the base structure.
 3. The tiltable stool as in claim 2,further comprising a plurality of mounting holes circumferentiallyspaced at the base structure through which radially outwardly directedscrews engage the upper section of the annular elastic base.
 4. Thetiltable stool as in claim 2, wherein a lower end of the annular elasticbase member contacts the base structure when the annular elastic base isfully deformed.
 5. The tiltable stool as in claim 1, wherein thedownwardly convex outer surface of a lower section of the annularelastic base member rests on the floor.
 6. The tiltable stool as inclaim 5, wherein a contact area between the annular elastic base memberand the floor is substantially ring-shaped, and wherein the contact areagrows outwardly when a weight placed on the seat.
 7. The tiltable stoolas in claim 1, wherein the annular elastic base member has asubstantially “j”-shaped cross section.
 8. The tiltable stool as inclaim 1, wherein the normal position is upright.
 9. The tiltable stoolas in claim 8, wherein the outer surface of an upper section of theannular elastic base member is substantially vertical.
 10. The tiltablestool as in claim 1, wherein the annular elastic base member comprises acircular tongue at its upper end which engages a corresponding circulargroove of a base structure.
 11. The tiltable stool as in claim 10,wherein the circular tongue is press-fitted into the correspondingcircular groove.
 12. The tiltable stool as in claim 10, wherein theannular elastic base is inserted into the circular groove of the basestructure by cooling the annular elastic base to reduce the size of thetongue.
 13. The tiltable stool as in claim 1, wherein the annularelastic base comprises thermoplastic polyurethane with a hardness ofabout 90 Shore.
 14. The tiltable stool as in claim 1, wherein theannular elastic base comprises a plurality of circumferentially spacedand radially extending grooves or ridges in a contact area with thefloor.
 15. A tiltable stool, comprising: a seat; a base comprising arotationally symmetrical annular elastic base member with asubstantially “j”-shaped cross section having a downwardly convex outersurface and a downwardly concave inner surface extending around acentral opening; and an elongated body structure extending between theseat and the base.
 16. The tiltable stool as in claim 15, wherein thetiltable stool has a substantially upright position when not subjectedto a tilting force, and wherein the tiltable stool can be tilted awayfrom the upright position into a dynamic seating position by a tiltingforce, and wherein deformation of the annular elastic base causes astabilizing force which pushes the tiltable stool against the tiltingforce from the dynamic seating position towards the upright position.17. The tiltable stool as in claim 16, wherein the stabilizing forceincreases with a tilt angle between the dynamic seating position and theupright position.
 18. The tiltable stool as in claim 17, wherein thestabilizing force increases approximately exponentially with the tiltangle.
 19. The tiltable stool as in claim 16, wherein the stabilizingforce increases with a weight that is applied to the seat.
 20. Thetiltable stool as in claim 15, wherein the annular elastic base memberis tapered, having a downwardly decreasing thickness of approximately 13mm at an upper end and of approximately 2 mm at a lower end, and whereinthe annular elastic base member is made of thermoplastic polyurethanehaving a hardness of approximately 90 Shore, and wherein the upper endof the annular elastic base member has a diameter of approximately 333mm, and wherein the lower end of the annular elastic base member has adiameter of approximately 245 mm, and wherein a stiffening ring having athickness of approximately 8 mm is provided at the lower end of theannular elastic base member.
 21. A tiltable stool, comprising: a seat; abase comprising a base structure and an annular elastic base memberhaving a downwardly convex outer surface and a downwardly concave innersurface, the outer surface and the inner surface extending from an upperend to a lower end, the upper end of the annular elastic base memberbeing firmly connected to the base structure; and a body structureextending between the seat and the base, wherein deformation of theannular elastic base causes a stabilizing force which pushes thetiltable stool towards a normal position when the tiltable stool orchair is tilted out of the normal position.
 22. The tiltable stool as inclaim 21, wherein the annular elastic base member has a diameter at theupper end that is between 1.2 and 1.6 times its diameter at the lowerend.